The present invention is related to the field of digital communication, and particularly to a method and means for decreasing the susceptibility of a transmitted and received sub-band coded data stream to channel errors in mobile communication systems without increasing the required bit rate.
In such systems, channel errors often arise during transmission due to noise, interference and fading of the signal. Resulting errors in the data stream can cause severe deradation in the quality of the recovered speech signals. In the prior art there are many approaches to the transmission of digital speech signals. Many of these are based on the characteristics of speech; i.e., the statistics of the frequencies included and the energies in those frequencies. For example, it is known to utilize sub-band coding with energy quantization for decreasing the susceptibility of transmitted speech signals to errors due to noise during transmission. Most such systems operate by sending two main types of information, energy codes representing the energy in each band, and the sample codes for the band signals.
Since it is not efficient to use redundancy or other types of protective coding for all of the bits transmitted, various ways of determining the most vulnerable parts of the bit stream have been tried. Some systems provide error detection and protection on all of the bits used for transmitting energy codes, usually termed "side information". One such system was described by Kaltenmeier and Proegler in a paper given at the first Nordic Seminar on Digital Land Mobile Radiocommunications, 5-7 Feb., 1985, at Espoo, Finland. In this system approximately 3.5 kbps of error protection were used to provide error protection for all energy information as well as some band sample information. The side information was reduced by transmitting a small number of band energies, thus reducing speech quality at the receiver/decoder.
In another paper, given by Aarskog/Hergum at the seminar referenced above, vector quantization of the band energies is used to reduce the amount of side information which should be protected. Vector quantization, although it reduces the bit rate required for the side information, requires the storage of a large codebook, and requires additional computations to find the best match of all the codebook vectors. The net result is that an additional delay is introduced into the system, depending on the number of computations which are required. Such delays, as well as the added complexity, may not be acceptable in some applications.
Dynamic bit allocation has been determined to be advantageous, due to the need for more dynamic range for those bands with the most energy content. The general effect of errors on the band sample information bits themselves is to cause some roughness in the reproduced speech, but little loss of intelligibility. In the side (energy) information, however, which is used to provide information as to the bit allocations, a bit error can have a much greater effect. The error is very likely to be propagated over many bands, destroying the receiver's ability to reproduce the bit allocations of the bands accurately.
It is not particularly difficult to solve any of these problems if there are unlimited bits available for "error protection". For instance, it is known to protect all of the bits being transmitted by simply duplicating the transmission in some manner or by providing an elaborate redundancy code. Errors in received signals can also be detected and, possibly, corrected in one of many ways. The difficult part is to protect and, if necessary, correct the signal to a high degree of accuracy with a minimum number of added bits.